As stated before, Aperture and PhotoShop are very different programs. PhotoShop is for image editing, while Aperture is for bulk image management. PhotoShop would be useful for tweaking, editing and modifying an image while Aperture is immensely useful for sorting through 1000 photos a day and keeping them in some meaningful order.
My impression of Aperture is that it is an awesome program for a professional photographer who averages at least 500 photos in a working day, uses multiple cameras, has a lot of different jobs to do, works with several models or events in a given day, and so on. For this sort of user, it is a godsend.
PhotoShop, by comparison, serves a very different purpose. Comparing the two is like comparing a sports car to an off-road vehicle - both do a different job.